1. Technical Field
The present invention relates to a human cooperative robot system which carries out work in the same workspace as a human. More specifically, the present invention relates to a robot system which monitors the contact force of a robot and a human and ensures the safety of the human.
2. Description of the Related Art
With respect to an industrial robot, in order to ensure the safety of humans, a safety fence is set up around the moveable area of a robot and humans are restricted from entering the area in which the robot can move. However, in recent years, instead of a safety fence, industrial robots on which certain measures have been carried out to ensure sufficient safety thereof to humans have allowed work to be carried out in a workspace shared by humans and robots. As a result, the desire for these types of robot, namely, human cooperative robots has increased.
By using a human cooperative robot system, separate work can be carried out by a human and a robot working in the same space, or a human can carry out work on a workpiece held by a robot. However, when using a human cooperative robot, when a human and a robot are sharing the same workspace, it is necessary to prevent injuries caused by a robot contacting the human. Accordingly, a method of monitoring the contact force between a human and a robot using a force sensor attached to a robot arm has been adopted. More specifically, if a contact force exceeding a predetermined threshold is detected by the force sensor, the operation of the robot is stopped, or the robot is moved in a direction to reduce the contact force.
Note that, in order to ensure the safety of a human, Japanese Patent No. 4650062 and No. 3067363, disclose a technology wherein the operation of a robot is controlled according to the temperature surrounding the robot.
Patent No. 4650062 discloses a robot controller which corrects the positioning accuracy of a robot by using temperature data from each of the temperature sensors provided on encoders of a plurality of motors which drive the robot.
Patent No. 3067363 discloses a moveable robot equipped with a plurality of sensors for detecting every type of abnormal state, in which one of the sensors is a temperature sensor, wherein the robot gives notification of an abnormality when the temperature measured by the temperature sensor exceeds a predetermined threshold.
In the aforementioned prior art, a force sensor for detecting the contact force comprises a force sensor body and an electrical resistance type strain gauge attached to the force sensor body. Further, the force sensor body is provided on a robot arm, the amount of strain on the force sensor body is detected by the strain gauge, and based on the detected value, the direction and size of the force applied to the robot arm are recognized. Accordingly, a material which is comparatively resilient to impact forces is selected as the material for the force sensor body to which the strain gauge is attached, and a metal is commonly used.
Therefore, the volume of the metal force sensor body can change due to the change in temperature of the surroundings of the robot. In such a case, the change in volume of the force sensor body caused by the temperature change is added to the amount of strain on the force sensor body caused by the force applied to the robot arm.
In other words, when the temperature of the surroundings of the robot changes from a first value to a second value, even if the same force is applied to the robot arm at each of the temperatures, the detected value by the force sensor will differ greatly between the first value and the second value.
Specifically, in a human cooperative robot system, when the contact force detected by the force sensor exceeds a predetermined threshold, the operation of the robot is stopped, or the robot is moved to reduce the contact force. Accordingly, to lessen bodily harm when contact is made with the robot, the threshold should ideally be set to as small a value as possible.
However, as described above, the value detected by the force sensor with respect to the same force, changes according to the temperature, and thus it is necessary to set the threshold while taking into account the change in the detected value caused by the temperature. Accordingly, there is the risk that the force sensor may not detect contact between a human and the robot after such an adjustment as the force applied to the robot which could previously have been detected by the force sensor, needs to be larger. As a result, there is an increased risk of the robot causing the human harm.
Note that Japanese Patent Nos. 4650062 and 306763 disclose a method of controlling the operation of the robot according to the temperature surrounding the robot, but the problem of detection accuracy of the contact force between the robot and the human is completely unrecognized.